Prep those veggies! Wash and dry all. For the bell peppers, cut off the top and bottom, then halve. Cut away the membrane and seeds, then cut into thin strips lengthwise. Peel the onion, halve, then cut into thin slices.
Sauté the veggies. In a large skillet, preferably nonstick, heat 2 tablespoons of olive oil and 1 tablespoon unsalted butter over medium heat. Add the peppers and onions, sprinkle all with salt and pepper, and sauté, stirring occasionally, for 7–10 minutes, until soft and golden brown.